Motion compensation system for watercraft connected to subsea conduit

ABSTRACT

A safety device  30  for compensating for ocean heave motion of a vessel or watercraft connected to a conduit  12  or drill string extending to the subsea. The safety device comprises at least one vertical piston and cylinder assembly  40,41,  the piston rod  40   a  of which connects to an upper beam  35  of the device  30  and the cylinder  40   b  of which connects to a horizontal beam  42  below the upper beam  35  and supported by vertical tension legs  51,52,  each including a gas expansion chamber  51   a,   52   a.  A valve controlled conduit system  61,62  connects the chambers  51   a,   52   a  and cylinders  40   b,   41   b  to a source of pressurized gas whereby a fixed volume of gas and gas pressure positions the pistons at the mid-lengths of the cylinders  40   b,   41   b  The safety device  30  is mounted on a frame or rig  11  mounted on the watercraft and the base  33  of the safety device  30  is connected to the bottoms of the tension legs  51,52.  A connecting means  34  connects the safety device  30  in coaxial alignment with the conduit or drill string  12  which extends through an opening  13  in the base  14  of the frame  11.  A second connector  24  connects the top of the safety device  30  to the vessel or watercraft or to another motion compensator used in series therewith.  
     When the conduit or drill string  12  stress is acceptable, the length of the safety device  30  remains constant. However, should the stress exceed acceptable levels, the pistons  40   a,   41   a  move into the cylinders  40   b,   41   b  to shorten its length and thereby reduce tension stress to an acceptable level or the pistons  40   a,   41   a  move outwardly of the cylinders to increase the safety device  30  length and thereby reduce the compression stress to an acceptable level.

FIELD OF THE INVENTION

[0001] The invention relates to systems for compensating for the motion imparted by the ocean heaves to floating drilling rigs or barges, and more particularly to a safety system for compensating for the ocean heave motion of a floating vessel or barge when connected to a conduit or drilling string extending to any sub-sea formation.

BACKGROUND OF THE INVENTION

[0002] Offshore vessels that deploy a fixed conduit to the ocean floor, oftentimes require a motion compensation system which serves to maintain the position of the conduit and also the levels of compression and tension imposed on the conduit within acceptable levels as the vessel follows the ocean movement up and down. In most circumstances and conditions, the traditional motion compensator is perfectly adequate. However, it is always possible that, regardless of circumstances, a failure of the motion compensator system can occur, in which instance the failed system “locks” the vessel to the conduit. This “locking” of the vessel to the conduit can occur at any part of the ocean heave cycle. When the conduit is fixed to the seabed, as during drill stem testing or completion operations and a “locking” of the vessel to the conduit occurs during the upward motion of the vessel in the ocean heave cycle or at the top thereof, a level of tension force may be imparted to the conduit which exceeds an acceptable safe level, and, conversely, a “locking” of the vessel to the conduit during downward motion of the vessel in the ocean heave cycle or at the bottom thereof can place the conduit under a compressive force which exceeds an acceptably safe level. In either event, whenever the force of compression or tension imparted to the conduit exceeds a predetermind safe level as is normally maintained by the traditional motion compensator, if this fails, catastrophic consequences may be the result. The present invention is intended to be used as the sole motion compensator to maintain the stress within acceptable limits or to be used in series with a traditional motion compensator as a safe-guard in the event of a failure of the traditional motion compensator.

[0003] The invention is a safety device for compensating for motion imparted to a floating vessel or barge by the ocean heaves when the vessel or barge is connected to a conduit or drill string extending to subsea formations and when the stress imposed on the conduit or drill string exceeds predetermined levels. The system comprises at least one vertically disposed piston and cylinder assembly, the piston rod(s) of which connect to an upper support beam of the device and each cylinder of which connects at its top to a horizontal beam below and parallel to the upper support beam. The beam which connects the top(s) of the cylinder(s) is connected by a pair of vertical tension legs, each of which includes a gas expansion chamber intermediate its ends. Pressurized gas is supplied to the gas chambers and cylinder(s) by a conduit system which includes a gas feed line with valve therein which connects one of the chadders to a source of pressurized gas whereby a fixed volume of gas and pressure is supplied to position the piston(s) substantially mid-length of the cylinder(s). If more than one piston and cylinder assent)y is provided, a gas line connects cacti cylinder at the top, thereof with the gas expansion chambers.

[0004] The safety device is Counted in a frame or rig having a base member, top beam and sides. The safety device includes a base which connects with the bottom ends of the tension legs and is provided with means for connecting the base of the safety device with the conduit (or drill string) which extends through an opening in the base of the frame or rig such that the safety device is coaxially disposed with the conduit (or drill string). A second connecting means connects the top beam of the safety device with a rigid member of the vessel or barge or with another motion compensator which may be used in series therewith.

[0005] Under conditions when the stress imposed on the conduit (or drill string) is within acceptable levels, the length of the safety device remains constant and the safety device acts as a rigid connection between the watercraft and the conduit (or drill string). However, should stress on the conduit (or drill string) exceed acceptable levels with vessel pitch and heave, the device responds by motion of the pistons inwardly of the cylinders to shorten its length and thereby reduce compression stress to an acceptable level or by motion of the pistons outwardly to increase the length of the safety device and thereby reduce the tension stress on the conduit (or drill string) and maintain it within an acceptable limit.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006]FIG. 1 is a schematic front view of a semi-submersible vessel having a conventional lifting frame (or derrick) thereon and a traditional motion compensator mounted on the frame and connected to the safety device of the invention which serves as an interface connection between the traditional motion compensator and a conduit which passes from the vessel to the ocean floor;

[0007]FIG. 2 is a schematic front view similar to FIG. 1 but showing an operative condition of the safety device of the invention wherein the pistons of the safety cylinders of the safety device have moved upward and thereby relieve tension on the conduit after the traditional motion compensator has failed at the top of an ocean heave cycle and placed the conduit under a tension force which exceeds the predetermined limt of tension force normally maintained by the traditional motion compensator; and

[0008]FIG. 3 is a schematic front view similar to FIG. 1 but showing an operative condition of the safety device of the invention wherein the pistons of the safety cylinders of the device have moved downward into the cylinders and thereby relieve the compression force on the conduit after the traditional motion compensator has failed when the rig is at the bottom of the ocean heave cycle and the compression force on the conduit has exceeded the predetermined level of compression normally maintained by the traditional motion compensator

DETAILED DESCRIPTION OF THE INVENTION

[0009] Referring more particularly to the drawings, FIG. 1 shows A partly sectional view of an offshore vessel in the form of a semi submersible rig 10 with a lifting frame 11 thereon and from which a conduit 12 is suspended and and extends to the ocean floor (not shown). The rig 10 includes a main deck or base 14 which is mounted on floats 16 a, 16 b affixed to the bottom of the base 14 at opposite sides thereof. The rig 10, however, might also be mounted on a vessel or ship in lieu of the floats 16 a, 16 b.

[0010] The structure of the lifting frame 11 includes side members 18 a, 18 b which extend upwardly from opposite sides of the base 14 in a generally convergent direction and support at their tops a top support beam 19 affixed thereto and mounted thereon in parallel relation to the base 14.

[0011] The rig 10 with lift frame 11 have served in the past to support and carry a state-of-the-art motion compensation system which has been traditionally used to maintain the position of the conduit which is fixed to the ocean floor and to maintain the forces of tension and compression as may be imposed on the conduit within acceptable limits. Such motion compensation systems for seagoing vessels are commercially available from drilling companies such as NATIONAL OILWELL.

[0012] The traditional motion compensation system 20 as shown in FIGS. 1-3, is typically mounted atop the frame 11 on the horizontal support beam 19 at the top of the frame and extends through an opening 17 in the support beam 19 to provide a moveable travelling block 22 from which a lower support member 24 is suspended on a pair of leg supports 24 a, 24 b attached to the underside of the travelling block 22. When the present safety device 30 of the invention is not used, the support 24 at the low end of the motion compensator 20 is normally fixed directly to the conduit 12 to the ocean floor,which conduit extends through an opening 13 in the base 14 of the rig 10.

[0013] The safety device 30 of the present invention is particularly designed for use on motion compensated floating drilling vessels as are used in offshore drilling in the oil industry The support 24, at the low end of the motion compensator 20 is connected directly to the upwardly extending connecting leg 31 of the safety device 30. The connecting leg 31 is designed with an external cylindrical surface substantially identical to the outer cylindrical surface of the conduit 12 and is connected as by a clamp, welding or threads to the support beam 24 in the same fashion as the conduit 12 would otherwise connect to the support 24 of the motion compensator 20 if the safety device 20 were not used.

[0014] Near its lower end, the safety device 30 is provided with a horizontal base 33 from which is suspended a pair of load bearing leg members 34 a, 34 b. A lower horizontal support beam 34 is fixed to the lower ends of the legs 34 a, 34 b and constitute the low end of the device 30. The legs 34 a, 34 b and support beam 34 are substantially identical with both load bearing legs 24 a, 24 b and lower support beam 24 from which suspends the travelling block 22 on the low end of the motion compensator 20.

[0015] The support beam 24 at the low end of the safety device 30 is connected to the upper end of the conduit 12 in the same fashion the conduit 12 normally connects to the lower support beam 24 of the motion compensator 20.

[0016] The safety device 30 includes an upper horizontal support beam 35 which is vertically su per ed by the piston rods of the pistons 40 a, 41 a of a pair of piston and cylinder assemblies 40,41. The cylinders 40 b, 41 b of the respective assemblies 40, 41 are fastened to and suspended from a horizontal support beam 42 which is parallel to the support beam 35 and the base 33. The cylinders 40 b, 41 b receive their respective pistons 40 a, 41 a. The support beam beam 42 is joined to the base member 33 of safety device 30 by a pair of parallel vertically extending tension legs 51, 52. The tension leg 51 includes an elongate gas expansion chamber 51 a located intermediate the ends of the leg 51 and configured in coaxial relation therewith. The tension leg 52 similarly includes an elongate cylindrical gas expansion chamber 52 a located intermediate the ends of the leg 52 and also configured in coaxial relation therewith.

[0017] Pressurized gas is supplied to the gas chamber 52 a by a gas feed conduit 61 which the chamber 52 a to a source of pressurized gas (not shown). An additional gas transfer conduit 62 connects with the upper end of the chamber 52 a and extends across the horizontal support beam 42 and downward to a fluid connection with the expansion chamber 51 a. The gas line conduit 62 is also provided with fluid connection with each of the cylinders 41 a, 41 b at the tops of the cylinders so as to pressurize the pistons 40 b 41 b in a downward direction in their respective cylinders when pressurized gas is in the chambers 51 a, 52 a. A valve 63 is installed in the gas feed line 61 whereby the connection with the source of pressurized gas or fluid can be opened to supply the chambers 52 a, 51 a. The valve 63 may be closed when the gas is at a predetermined level of pressure. At this pressure level, the pistons 41 a, 40 a are positioned substantially at the mid-length points of their respective cylinders 41 b, 40 b.

[0018] It is to be appreciated that so long as the traditional motion compensator 20 maintains its normal operation, the safety device 30 of the present invention is substantially inoperative in the sense that the pistons 40 a, 41 a do not stroke but maintain their positions in the safety cylinders 40 b, 41 b. In such normal operation of the motion compensator 20, the length of the safety device 30, that is, the distance between its connection with the motion compensator 20 at the support 24 and its connection with the conduit 12 at the support 34 remains constant and the safety device 30 acts as a rigid interface connection between the motion compensator 20 and the conduit 12. However, should a failure of the motion compensator 20 occur, as for example, when the vessel or rig is at the trough of an ocean heave movement, undue stress of compression on the conduit 12 is likely to produce catastrophic results unless corrected. If such failure occurs, the safety device 30 responds when the tension of the conduit 12 exceeds the predetermined level of tension at which it is normally maintained by the motion compensator 20. The safety device 30 responds by movement of the pistons 40 a, 41 a outward of their safety cylinders as shown in FIG. 2 to, in effect, increase the length of the safety device 30 and exert a pull on the conduit 12 to a degree which relieves its tension stress to within a predetermined limit controlled by the safety device 30. The predetermined limit of tension maintained by the safety device 30 is incrementally higher, for example, one percent higher than that maintained by the traditional motion compensator 20.

[0019] On the other hand, if a failure of the motion compensator 20 occurs when the vessel or rig is at the top of an ocean heave cycle, an undue level of compression on conduit 12 may also produce catastrophic results unless corrected by the safety device 30. When such a failure occurs to produce compressionstress on the conduit 12 above the level at which it is normally maintained by the motion compensator 20, the safety device 30 responds by movement of the pistons 40 a, 41 a to, in effect,shorten the length of the safety device as the pistons move into the cylinders and thereby exert a force to relieve the excessive compression on the conduit 12 and to maintain the compression on the conduit 12 within an acceptable predetermined limit which is incrementally greater, for example, one percent greater than the limit of stress normally maintained by the traditional motion compensator 20.

[0020] It is to be noted that as the pistons 40 a, 41 a move out of the cylinders when relieving tension stress on the conduit 12, cylinder gas is transferred to the gas expansion chambers 51 a, 52 a as seen in FIG. 2. When the alternative occurs and the pistons move into the safety cylinders as seen in FIG. 3, pressurized gas is transfered from the expansion chambers 51 a, 52 a to the cylinders 40 b, 41 b.

[0021] It is also to be appreciated that a failure of the motion compensator 20 might occur at any part of the ocean heave cycle, but the safety device 30 responds as necessary to relieve excessive tension on the conduit 12 within the incrementally higher predetermined limit of tension as controlled by the safety device 30.

[0022] Furthermore, the safety device 30G has been shown and described herein as being attached to a traditional motion compensator 20 and to be responsive in the event of a failure of the compensator 20 but, it may, however, be employed as the only motion compensator provided on the vessel or barge for maintaing the 35 levels of tension and compression imposed on the conduit 12 within acceptable limits. In such event, the safety device 30 is connected to a rigid connector 20 part of the vessel in lieu of the compensator 20

[0023] It is also to be appreciated that while the safety device 30 of the invention is heretofore described as having a pair of gas expansion chambers 51 a, 52 a and a pair of piston and cylinder assemblies 40, 41, it could be made with only one piston and cylinder assembly located on the axis 50 of the safety device 30 and in coaxial alignment with the conduit 12 to the ocean floor. It might also be made with more than a pair of such piston and cylinder assemblies, preferably located symmetrically with respect to the axis 50. Furthermore, the gas expansion chambers need not be elongate in form and need not be in coaxial alignment with the tension legs 51, 52, but could be located elsewhere. It is to be understood therefore, that the fore-going description of a preferred embodiment of the invention has been presented for purposes of explanation and illustration and is not intended to limit the invention to the precise form disclosed. Accordingly, it is to be understood that various changes may be made to the safety device 30 in structure and materials without departing from the spirit of the invention. 

What is claimed is:
 1. A safety device for compensating for the ocean heave motion of a floating watercraft connected to a conduit extending downwardly to beneath the ocean floor in order to maintain the stress imposed on the conduit by the watercraft to within acceptable levels, said safety device comprising: at least one piston and cylinder assembly disposed vertically with the end of the piston rod connected to an upper beam of the device and the cylinder connected to a horizontal support beam below said upper beam, said horizontal support beam supported by a pair of vertically extending tension leg members, each of which includes a gas expansion chamber therein; a valve-controlled conduit system connecting the gas expansion chambers and said cylinder to a source of pressurized gas whereby a fixed volume of gas and gas pressure may be supplied to said cylinder and piston to position the piston substantially at the mid-length of the cylinder, said safety device having a base member connected to the bottom ends of said tension leg members and said safety device being mounted in a frame mounted on said watercraft and having a frame base provided with an opening therein through which said conduit extends; a first connecting means for connecting said base member of said safety device to the upper end of said conduit which extends through said opening in said frame base; a second connecting means for connecting said upper beam of the safety device to a connection member provided on said floating watercraft, said safety device being adapted to respond to stress on said conduit which exceeds a predetermined level by movement of said piston relative to said cylinder to alter the length of said safety device in a direction to relieve said stress and to maintain the stress within an acceptable limit.
 2. A safety device as set forth in claim 1 wherein said safety device is provided with more than one piston and cylinder assembly with the piston rods thereof connected to said upper beam and the cylinders of said assemblies connected to said horizontal support beam and to said conduit system whereby a fixed volume of gas and gas pressure may be supplied to said cylinders to position the pistons substantially at the mid-lengths of said cylinders.
 3. A safety device as set forth in claim 1 wherein said safety device is adapted to respond to a level of tension stress on said conduit which exceeds a predetermined level by movement of said pistons outwardly of said cylinders to thereby increase the length of said safety device and thereby relieve and maintain said tension stress within an acceptable level.
 4. A safety device as set forth in claim 2 wherein said safety device is adapted to respond to a level of tension stress on said conduit which exceeds a predetermined level by movement of said pistons outwardly of said cylinders to thereby increase the length of said safety device and thereby relieve and maintain said tension stress within an acceptable level.
 5. A safety device as set forth in claim 1 wherein said safety device is adapted to respond to a level of compression stress on said conduit which exceeds a predetermined level by movement of the pistons inwardly of said cylinders to thereby decrease the length of said safety device and thereby relieve and maintain said compression stress within an acceptable limit.
 6. A safety device as set forth in claim 2 wherein said safety device is adapted to respond to a level of compression stress on said conduit which exceeds a predetermined level by movement of the pistons inwardly of said cylinders to thereby decrease the length of said safety device and thereby relieve and maintain said compression stress within an acceptable limit.
 7. A safety device as set forth in claim 1 wherein said watercraft and said safety device are connected to a drilling string and said safety device is connected thereto in order to maintain the stress levels imposed on said drilling string by movement of said watercraft by the ocean heave movements to within acceptable limits.
 8. A safety device as set forth in claim 2 wherein said watercraft and said safety device are connected to a drilling string and said safety device is connected thereto in order to maintain the stress levels imposed on said drilling string by movement of said watercraft by the ocean heave movements to within acceptable limits.
 9. A safety device as set forth in claim 1 wherein said second connecting means connects said upper beam of the safety device to a traditional motion compensator provided on said watercraft and whereby said safety device becomes operative to relieve and maintain stress imposed on said conduit within acceptable limits in the event of failure of the traditional motion compensator. 